Respiration humidifiers are necessary during the artificial respiration of patients, in whom the function of the nose is bridged over by a tube. The inspired air must therefore be humidified and optionally heated to physiological values during artificial respiration to preserve the lung function.
Inhalative sedation or anesthesia of patients is necessary for various medical applications, e.g., during surgery. Anesthesia reflectors are used to transfer the anesthetic from the expiration gas breathed out by the patient to the inspiration gas to be breathed in by the patient (reflection) and thus to make multiple utilization possible and not to increase the concentration of anesthetic in the room in which the patient is located. Furthermore, the anesthesia reflector shall humidify and optionally heat the inspired air to physiological values.
Humidity sensors are used in respiration humidifiers and anesthesia reflectors in order to determine the humidity in the breathing air, i.e., the inspired gas and/or the expired gas. The data measured by the humidity sensors are used to control and regulate the humidity of the breathing air.
The use of humidity sensors with inexpensive measurement methods, e.g., a capacitive humidity measurement, is too inaccurate. The use of the dew point-mirror principle of measurement is too expensive for application in respiration humidifiers. Such sensors are, moreover, arranged in the breathing air, i.e., in a gas channel or a humidifying chamber, so that there are bacteriological problems.
U.S. Pat. No. 5,468,961 shows a humidity sensor for a device for respirating patients. Breathing air, whose humidity is to be determined, is sent through a sample cell. An infrared light source generates infrared light, which is sent through the sample cell and is subsequently detected by a detector. The humidity contained in the breathing air absorbs the infrared light, so that the quantity of infrared light detected by the detector decreases with increasing humidity in the breathing air and vice versa. The manufacture of the infrared light source, detector as well as sample cell is expensive. Furthermore, complicated means are needed to avoid the condensation of water on the inner surface of the sample cell.